How can I find the number of years between two dates?

import java.util.Calendar;
import java.util.Locale;
import static java.util.Calendar.*;
import java.util.Date;

public static int getDiffYears(Date first, Date last) {
    Calendar a = getCalendar(first);
    Calendar b = getCalendar(last);
    int diff = b.get(YEAR) - a.get(YEAR);
    if (a.get(MONTH) > b.get(MONTH) || 
        (a.get(MONTH) == b.get(MONTH) && a.get(DATE) > b.get(DATE))) {
        diff--;
    }
    return diff;
}

public static Calendar getCalendar(Date date) {
    Calendar cal = Calendar.getInstance(Locale.US);
    cal.setTime(date);
    return cal;
}

Note: as Ole V.V. noticed, this won't work with dates before Christ due how Calendar works.

tl;dr

ChronoUnit.YEARS.between( 
    LocalDate.of( 2010 , 1 , 1 ) , 
    LocalDate.now( ZoneId.of( "America/Montreal" ) ) 
)

java.time

The old date-time classes really are bad, so bad that both Sun & Oracle agreed to supplant them with the java.time classes. If you do any significant work at all with date-time values, adding a library to your project is worthwhile. The Joda-Time library was highly successful and recommended, but is now in maintenance mode. The team advises migration to the java.time classes.

Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport and further adapted to Android in ThreeTenABP (see How to use…).

LocalDate start = LocalDate.of( 2010 , 1 , 1 ) ;
LocalDate stop = LocalDate.now( ZoneId.of( "America/Montreal" ) );
long years = java.time.temporal.ChronoUnit.YEARS.between( start , stop );

Dump to console.

System.out.println( "start: " + start + " | stop: " + stop + " | years: " + years ) ;

start: 2010-01-01 | stop: 2016-09-06 | years: 6

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

I would recommend using the great Joda-Time library for everything date related in Java.

For your needs you can use the Years.yearsBetween() method.